Since the 1940s IOLs have been utilized as replacements for diseased or damaged natural ocular lenses. In most cases, an IOL is implanted within an eye at the time of surgically removing the diseased or damaged natural lens, such as for example, in the case of cataracts. For decades, the preferred material for fabricating IOLs was poly(methyl methacrylate), which is a rigid, glassy polymer.
As the technology in IOL materials progressed, softer, more flexible IOLs have gained acceptance because of their ability to be compressed, folded, rolled or otherwise deformed. As a result, the IOLs can be deformed prior to insertion of the lens through an incision in the cornea and into the eye. Once inserted, the IOL is carefully unfolded by the surgeon and the lens returns to its original pre-deformed shape. These softer IOLs can be inserted into an incision of less than 3.0 mm, whereas the earlier, more rigid IOLs required an incision size of 5 to 7.0 mm, i.e., an incision size slightly larger than the diameter of the optic portion of the IOL. Since larger incisions lead to an increased incidence of postoperative complications, the softer, more flexible IOLs are typically preferred by ocular surgeons.
The refractive power of a lens is a function of its shape and the refractive index of the material of which it is made. Accordingly, a lens made from a material having a higher refractive index can be thinner and provide the same refractive power as a lens made from a material having a relatively lower refractive index. For IOLs designed to be rolled or folded for insertion through a small incision, a lens of thinner cross section is inherently more flexible and can be rolled or folded to a smaller cross section.
In general, the materials of current commercial IOLs fall into one of three general categories: silicones, low-water hydrophilic acrylics and hydrophobic acrylics. Hydrophobic acrylic materials with a relatively low glass transition temperature (Tg ° C.) possess important material characteristics—they typically have a high refractive index and unfold with a greater degree of control. Low Tg, hydrophobic acrylic materials contain little or no water at the point of manufacture, however they do absorb small amounts of water over time if stored in an aqueous medium or following ocular insertion. The absorbed water often leads to pockets of water or vacuoles within the polymeric matrix, which leads to a reduction in the visual quality of the lens. Low Tg, hydrophobic acrylic materials also are known to develop disc-like features, which also lead to a reduction in the visual quality of the lens. The use of low water, hydrophilic acrylics minimizes the formation of these disc-like features and vacuoles.
Accordingly, there is an interest to develop a manufacturing process that minimizes the formation of the disc-like features and vacuoles in hydrophobic acrylic and low water, hydrophilic acrylic IOLs.